Clinical Report: Hot tub lung may mimic refractory pneumonitis

Overview

A 66-year-old woman with persistent respiratory and imaging abnormalities despite more than 2 years of treatment for presumed hypersensitivity pneumonitis improved after daily indoor hot tub exposure was identified, hot tub use stopped, and antimycobacterial therapy began. Biopsy findings and rare MAC organisms supported hot tub lung, although negative BAL culture and the absence of hot tub water or filter cultures limited confirmation of active infection and its source.

Background

Hot tub lung is associated with exposure to aerosolized Mycobacterium avium complex (MAC) and may involve immune-mediated inflammation, infection, or both. Its clinical and radiologic features can overlap with hypersensitivity pneumonitis and other interstitial lung diseases. The supplied case report notes that exposure avoidance may be sufficient in many reported cases, while corticosteroids or antimycobacterial therapy have been used in selected cases. The 2025 Fleischner Society consensus identifies hot tub lung as a distinct radiologic and pathologic entity rather than a generic subtype of hypersensitivity pneumonitis.

Data Highlights

Key Findings

• The patient’s daily indoor hot tub exposure was identified only after persistent disease despite treatment for presumed hypersensitivity pneumonitis.
• CT findings included centrilobular ground-glass nodules and mosaic attenuation; biopsy showed airway-centered and airspace granulomas with organizing pneumonia and chronic cellular interstitial infiltrates.
• Rare MAC organisms were seen in biopsy tissue, but BAL culture was negative, and hot tub water and filter components were not cultured; the authors therefore described definitive evidence of active infection as limited.
• After hot tub cessation, immunosuppression taper, and initiation of azithromycin, ethambutol, and rifabutin, the patient’s symptoms markedly improved and imaging resolved approximately 24 months after presentation.
• The case authors describe hot tub lung as potentially involving immune-mediated and infectious mechanisms; they report that exposure elimination may be sufficient in most reported cases and that antimycobacterial therapy is generally reserved for selected patients with evidence suggesting MAC infection or failure to improve with exposure avoidance and corticosteroids.
• The 2025 Fleischner Society consensus, as summarized in the supplied web research context, designates hot tub lung as a distinct radiologic and pathologic entity rather than a generic subtype of hypersensitivity pneumonitis.

Clinical Implications

The case authors state that persistent or treatment-refractory interstitial lung disease may warrant reassessment of the presumed antigen source and disease mechanism, including targeted questioning about hot tub exposure. They describe antimycobacterial therapy as generally reserved for selected cases with evidence suggesting MAC infection or inadequate improvement despite exposure avoidance and corticosteroid treatment.

Conclusion

This case report describes resolution of persistent lung abnormalities after hot tub exposure cessation and MAC-directed therapy, with biopsy findings supporting hot tub lung but microbiologic confirmation remaining limited. The authors emphasize targeted exposure history when evaluating persistent interstitial lung disease.

Related Resources & Content

1. Abualhommos et al., Respirology Case Reports, 2026 -- Hot Tub Lung: Misdiagnosed Hypersensitivity Pneumonitis Revealing a Mycobacterial Aetiology—A Case Report and Literature Review
2. Fleischner Society, American Journal of Respiratory and Critical Care Medicine, 2025 -- Standardized Clinical Terms and Definitions for Interstitial Lung Disease: A Consensus Statement from the Fleischner Society
3. Marras et al., CHEST -- Mycobacterium Avium Complex Hot Tub Lung: Infection, Inflammation, or Both
4. Infection — Management of Paradoxical Inflammatory Responses in Pulmonary Tuberculosis Patients Using Corticosteroid Therapy
5. ASCO Publications — Association of anti-PD-1 (PD1) induced pneumonitis with persistent imaging and lung function abnormalities in some patients.
6. ASCO Publications — Association of anti-PD-1 (PD1) induced pneumonitis with persistent imaging and lung function abnormalities in some patients.
7. MDSpire News — COVID Scars: Lung Abnormalities 2 Years After Hospitalization
8. Standardized Clinical Terms and Definitions for Interstitial Lung Disease: A Consensus Statement from the Fleischner Society
9. Mycobacterium Avium Complex Hot Tub Lung: Infection, Inflammation, or Both
10. Hot Tub Lung: Misdiagnosed Hypersensitivity Pneumonitis Revealing a Mycobacterial Aetiology—A Case Report and Literature Review
11. Standardized Clinical Terms and Definitions for Interstitial Lung Disease: A Consensus Statement from the Fleischner Society | American Journal of Respiratory and Critical Care Medicine | Oxford Academic
12. Mycobacterium Avium Complex Hot Tub Lung: Infection, Inflammation, or Both - CHEST
13. Hot Tub Lung: Misdiagnosed Hypersensitivity Pneumonitis Revealing a Mycobacterial Aetiology—A Case Report and Literature Review - Abualhommos - 2026 - Respirology Case Reports - Wiley Online Library